By Sky News US Team
The Islamic State says one of its hostages, a 26-year-old American woman, has died in a Jordanian airstrike, according to the Site Intelligence Group, which monitors extremists.
The militants claimed aid worker Kayla Jean Mueller was killed in rocket fire from warplanes striking a building in which she was being held in Syria.
No fighters died in the raids, according to the terrorist group, which it said happened on Friday in their de facto capital of Raqqa.
Jordan said its jets carried out a second consecutive day of strikes on Islamic State sites on Friday, after the militants burned to death a captured Jordanian pilot.
But Jordan's interior minister dismissed the Islamic State's claim.
Foreign Minister Nasser Judeh tweeted: "An old and sick trick used by terrorists and despots for decades: claiming that hostages human shields held captive are killed by air raids."
Mueller, of Prescott, Arizona, disappeared in August 2013 in the northern Syrian city of Aleppo.
The Islamic State said: "The crusader criminal coalition bombed the location just outside of Raqqa as people were at Friday prayers for more than one hour.
"Allah has made their efforts fail and not a single mujahid (militant) was killed in the attack.
"But we can confirm the death of an American hostage by the rockets which targeted that area."
The group released images showing a damaged building it said had been targeted in airstrikes, but no photos of the hostage.
The White House, State Department and Pentagon said they could not confirm the unsubstantiated report.
"We are obviously deeply concerned by these reports," said Bernadette Meehan, spokeswoman for the White House National Security Council, in a statement.
"We have not at this time seen any evidence that corroborates ISIL's claim."
The aid worker's parents, Carl and Marsha Mueller, said in a statement on Friday: "Kayla found this (aid) work heart-breaking but compelling; she is extremely devoted to the people of Syria.
"When asked what kept her going in her mission, she said, 'I find God in the suffering eyes reflected in mine, if this is how you are revealed to me, this is how I will forever seek you.'"
After Mueller was abducted, her family received a proof-of-life video, showing her pleading for her life.
A media blackout on her plight was broken by President Barack Obama on Monday, when he confirmed the Islamic State had at least one US hostage.
In an interview with NBC, he said: "We're deploying all assets that we can, working with all the coalition allies that we can to identify their locations.
"We're in very close contact with her family, trying to keep them updated."
The President was speaking after the Islamic State released a video showing the execution of Japanese journalist Kenji Goto.
The group has already executed three American hostages: James Foley, Peter Kassig and Steven Sotloff.
British reporter John Cantlie is still being held captive by the militants. They executed two other Britons, David Haines and Alan Henning.
